karolh

After each flight my model shows signs on unburnt fuel on the inside and outside of the cowl at the bottom and right side facing front, and on the outside of the fuse rear of the carb on my G23 powered Goldberg Extra.

I have been told that adding a carb stack will solve the problem, but I would like a clear explanation of what is causing the fuel blowby and how does adding a stack stop it.

Will adding a carb stack require the engine to be retuned ?


Karol

Tired Old Man

My Feedback: (1)

The fuel residue you have is caused by "siphoning". What happens is the air flow overer the carb opening creates a low pressure area, drawing some of the fuel out of the carbs venturi. Adding a velocity "stack" will leave you with the same opening, only a little farther away from the carb then it was before. In short, fuel will still siphon, but to a lesser extent.

To eliminate the siphoning, you would need a stack that faced into the wind, acting like ram air. That would direct the entire air flow into the carb.

Big_Bird

My Feedback: (1)

With a common piston ported engine, on the compression stroke low pressure in the crank case pulls the fuel/air mixture into the crank case. As the piston starts back down the fuel/air mixture starts to compress. Just before the piston covers up the intake port, a small amount of the fuel/air escapes carrying a very small amount of fuel and oil which accumulates in the cowl and on the side of the plane.

The reason that a carburetor tube (velocity stack) works is that when the fuel/air mixture is trying to escape, it can't travel the distance of the tube length before it turns around and gets sucked back into the carburetor on the next cycle. The prevention of the loss of fuel/air mixture will happen no matter if the tube has a slanted tip or a straight tip. It will work if the slant is pointed forward or backward. Sometimes you have to experiment with the direction that you point the slant to get the best response from the engine. When using a carburetor tube, you may need to lean the engine a little because of the gasoline that you are saving.

You don't have this problem with a reed valve engine because the reed closes the moment that the fuel/air mixture stops going into the crank case.

Ken

rmh

There is some slop over in any system reeds -rotary/drum /piston port -
The reeds seat once the pressure differential is collapsed in the crankcase.
However
There is still airflow in motion inside the carb and the carb inlet which is parallel to a stream of air can get a problem -especially at very low engine speeds . (pressure differences again)
The bell shaped or a straight or trumpet shape may help
A good free flowing filter is also good at this
Some carbs have high speed orifices positioned such that spitting out the carb is an additional problem.
Siphoning as mentioned is a cause of spitting back -but not the only cause.
Ideally -the flow of air into and thru a carb would be a smooth constant flow
it ain't.

karolh

I might try building one as a DIY project, and am assuming that the I.D should be of a smiliar size as the carb's inlet and be approx 1.5" long. Are these reasonable assumptions.

I have seen pictures of both straight sided and bell shaped stacks, is there any advantage to the shape used, and how are they attached to a Walbro carb.

karolh

The main reason for my concern is that the on/off switch for my electronic ignition system which is a standard Rx switch, is mounted on the cowl and the switch gets hit on occasion with fuel from the carb spitting, which I'm afraid might damage the switch, or worst start a fire.

Karol

Big_Bird

My Feedback: (1)

It is easy to make a straight sided one. A bell shaped one could be a little more involved.

Get a thin piece of sheet brass or steel. This plate will fit over the carb intake and will be mounted to it using the carb mounting screws. Get a piece of 5/8" I.D. x 1.5" brass tubing. Cut a hole centered between the two mounting holes on the plate to accept the brass tubing. Solder the tubing to the plate.

Or

You can get one here for $8.95 plus shipping that will easily turn in any direction.

http://bennettbuilt.com/page2.htm

Ken

karolh

Could having my on/off switch located in the cowl and so close to the carb inlet possibly cause a fire.

flier

My Feedback: (7)

The way a carb stack works, is designed by its shape.
The straight tube with the angle cut tip, for use like an air scoop, the incoming air reduces the reverse flow of fuel/air in the venturi.
The trumpet shape is designed to smooth out and there by increase the air flow into the venturi.
These work to provide a denser fuel /air charge per intake cycle.
This why thier called "velocity stack" for the increased velocity through the venturi.
Use either of these in a reverse air flow environment and they will also have "siphoning" (to a lesser extent than without the stack).
Baffles / air dams work well at stopping this.
Think of the air flow like water, where will it go if coming from the cowl inlet?
How will it flow around the carb?
Some times an extension to place the incoming air in a better environment is all thats needed.

karolh

Thanks to you all for your indepth explanations of just what causes the fuel siphoning from a carb and how the use of a velocity stack helps to solve this problem.

I am still anxious to know if I should relocate my on /off switch from the cowl as a possible fire hazzard.

Dr1Driver

While those are all good answers, most are missing a few of vital points.

1. This is NOT a glow engine with a suction carb. This is a gas engine with a diaphragm-operated carb. There is very little "escaped" gasoline from the carburetor.

2. This is also NOT a reed-induction engine. Again, it uses a diaphragm carb with a side-induction system.

3. A velocity stack operation is very simple. The airstream flowing across the carb opening does siphon fuel. The velocity stack lessens the effect by moving the opening farther away from the gas inlet. The shape of the stack has very little effect on this. Most commercially made velocity stacks are 1" to 1 1/2" long.

The ideal orientation for the carb opening would be towards the rear of the plane.

An unsealed switch in the presence of gasoline vapor may indeed cause a fire. Either move your switch to the interior of the plane or use a explosion-proof (that's a fancy term for sealed) switch.

Dr.1

karolh

Just as I thought, regarding the possibility of starting a fire with gasoline vapour and an exposed switch in close proximity of each other.

Though I have had no problems so far, why tempt fate.

bob_nj

My Feedback: (62)

Is there a tutorial, or animation of any sort on the working of the reed valve engines that we use?
I'm not an engineer or anything, but do have a basic understanding of how things work.
I was just wondering if there is anything like this --> [link=http://www.keveney.com/]Animated Engines[/link]
for engines like 3W, DA etc?

Thanks and take care_bob

Big_Bird

My Feedback: (1)

I would like to comment on some of your points.
1. A Walbro gasoline carburetor IS a suction carburetor with respect to the way that the fuel is metered through the series of holes in the venturi as the throttle plate is opened. The pump diaphragm does not force fuel into the air stream, it only supplies the fuel to the needle and seat. The opening of the needle and seat is controlled by the atmospheric diaphragm.

2. You are right, a piston ported engine (which spits) is not a reed induction engine. Reed induction engines, such as a lot of the twins that we see (Brison, BME, 3W), are less prone to spitting because the pyramid reed bank closes so quickly.

3. If you want to prove to yourself that siphoning of the fuel is not the culprit but rather the explanation that I gave near the beginning of this thread then you might try an experiment. Cut out a piece of card board large enough to block most of the air flow from going across the inlet to the carb. Rubber band this card board to the front (prop side) of the carb. Start the engine and run it up in power. You can put your hand near and behind the inlet of the carb and you will see that a mist will leave oil on your hand. This is not due to siphoning action. If siphoning were the problem then why doesn't air blowing across the carb intake of a reed induction engine do the same thing? On my Brison 6.4 twin with a pyramid reed bank I have no problem with oily residue on the plane and there is no velocity stack on the engine. I assure you that there is high speed air flow across the carb. On the other hand, I had a lot of oily residue down the side of my G-62 powered Giant Aeromaster until I added a velocity stack.

Dr.1, this is not meant to be argumentative but to simply express a different of opinion.

Ken

JohnnyJ

My Feedback: (1)

Hello,
Someone else posted this info in another thread so I can't take the credit for it.
Very interesting info relating to velocity stacks. RC Showcase has some pretty nice bellmouth velocity stacks or intake trumpets.

WRK

My Feedback: (2)

Actually a fair amount but not all of the fuel spit back seen at the carb opening is caused by the reeds/piston port closing while there is still a column of air moving through the intake system, when the door is slammed shut this column bounces back toward the carb opening. If this action is watched with a strobe light it is interesting to see this column march froward and back at different engine speeds and frequencies, each bounce past the venturi picks up extra fuel. This is also one reason why many engines go rich at mid throttle, the air moving back and forth more than once per cycle accross the venturi causes more fuel to be drawn into the intake passage, since the high speed needle is set for wide open running this bouncing when it occurs can many times cause the mid range to go richer than it should.

Third port/piston port engines are usually worse for this condition but some reed valve engines can really spit back at certain engine/load speeds.

Bill Krueger

Rodney

If you use a stack with a slanted end, you will usually get best results with the slant facing aft, NOT RAM AIR. Of course if the cowl extends over the stack, you don't know for sure what direction to slant it as the air will be quite turbulent under the cowl depending on inlet and exit geometry. If the slant faces into the airstream, you will have a hard time getting consistant needle valve settings that match on the ground with what you get in the air. If you have the inlet inside the cowl, you may have to experiment for best position.

Flypaper 2

Many chainsaws use velocity stacks for the same reasons and have been for a very long time. Earlier snow machines used them too. Ask the old farts like me whos snowsuit smelled like an oil refinery

Kfoss1

If you buy one check it from time to time. Most are aluminum and the bell or stack is press fit into the mounting plate. I had one that became loose and was vibrating and turning spitting black goop all over inside the engine box. I tried to fix it with JB Weld but that didn`t last. I found a nice solid plastic stack at Tower and the problem was solved. It could be one of those hard to find radio glich vibrations too.

karolh

Thanks for passing on that piece of information. Based on it's contents it seems that the dimensions and shape of the stack is important to achieve optimum performnce when used as a velocity stack rather than just an aid to help stop fuel siphioning.

Rather than fooling around trying to reinvent the wheel, I have ordered a stack from B&B for my engine.

Karol

karolh

I have relocated the ignition switch from the cowl to inside the fuse as suggested, but on reflection I must have suffered a brain fart to have put it so close to the carb in the first place.

Though the cowl and fuse still gets wet from fuel siphioning as I have not yet installed a velocity stack, I feel so much more comfortable with this arrangement, plus knowing why the fuel blow-by happens.

Thanks to all who contributed to my query.

Brian Roth

I just finished breaking in a new DA 50. Is it alright to remove the carb bolts and install a velocity stack using the same seals? Or do I need to replace the seals after removing the bolts?

av8tor1977

My Feedback: (6)

Without a velocity stack or air filter, a fuel/air mixture will naturally spit back out of the carb. Some is also lost to a "siphoning" or venturi effect from air blowing over the carb entrance, but the majority of it is due to "spit back". This is much less, as has been mentioned, with reed valve engines. This phenomenon has been explained sufficiently here in this thread. It even has a name, it is called "stand off" or "reversion" and is common to all two stroke engines.

Brian Roth: The gaskets are usually ok to reuse if they don't tear upon removal, but it is a good idea to re-install them exactly as they were. You may need longer carb bolts however, to accommodate the velocity stack.

AV8TOR

Brian Roth

av8tor1977: Thanks a bunch. That was just what I needed to hear. Im on it!